Radio Frequency Identifier (RFID) tags are used in a variety of applications, such as inventory control and security. The advantage of these more intelligent RFID systems is that, unlike barcode tracking systems, an RFID system can store specific information about an article and can read that information on a tag without requiring line of sight or a particular orientation. This means that RFID systems can be largely automated, reducing the need for manual scanning.
These RFID tags are typically placed on or in articles or containers such as cardboard boxes. The RFID tags work in conjunction with an RFID base station. The base station supplies an electromagnetic wave output, which acts as the carrier frequency. Data are then used to modulate the carrier frequency to transmit specific information. RFID systems typically operate at either a low frequency range (generally less than 100 MHz), or a higher frequency range (greater than 100 MHz). In many applications, one such higher frequency range is between 800 and 1000 MHz, with 915 MHz being the most common high frequency currently utilized in the United States. Most RFID systems utilize frequency hopping centered around this frequency, such that the overall frequency range is approximately 902 to 928 MHz. A second high frequency used by RFID tags in the United States is 2450 MHz. Currently, European standards utilize 869 MHz and the Japanese standard is 953 MHz.
Many RFID tags contain integrated circuits, which are capable of storing information. Depending on the specific implementation of the RFID tag, the integrated circuit may be capable of replacing stored information with new information at a later time. When the base station requests data, the integrated circuit supplies the information that it has stored. In those RFID tags that permit information to be rewritten, the integrated circuit overwrites its existing information when new data are received from the base station.
In addition to the integrated circuit, the RFID tags include an antenna. This antenna is needed to receive the electromagnetic waves generated by the base station, and to transmit data via the same frequency. The configuration of the antenna can vary, and includes flat coils, patches, microstrip antennas, stripline antennas and dipoles.
Some of these RFID tags are self powered, that is, they contain an internal power supply such as a battery. Other RFID tags are field powered. These tags use incident RF energy transmitted by the base station to supply their required voltage. The RF energy is received by the tag antenna as an AC signal, which is then rectified to form a DC voltage, which is used to power the integrated circuit.
These integrated circuits have a minimum voltage requirement below which they cannot function and the tag cannot be read. The rectified DC voltage is a function of the signal strength of the received electromagnetic wave. For example, a RFID tag that is proximate to the base station will receive more energy and therefore be able to supply sufficient voltage to its integrated circuit, as contrasted to a RFID tag that is physically farther away from the base station. The maximum distance between the base station and the RFID tag at which the RFID tag can still be read is known as the read distance. In some applications, such as secure transactions, it may be preferable that the read distance be limited, such as less than 2 feet. These shorter read distances are typically achieved by using lower frequencies, preferably 13.56 MHz.
Passive RFID tag systems operating in the HF frequency band at 13.56 MHz employ magnetic induction to couple the transponder tag and the reader. The power required to energize and activate the tag microchip is drawn from the oscillatory magnetic field created by the reader. In free space, this magnetic field is undisturbed. However, if the RFID tag is placed on or in a metallic surface, the magnetic flux through the metal substrate induces eddy currents within the metal that oppose the reader's magnetic field. This damps the magnetic field in the metal to such a degree that communication between reader and transponder may no longer be possible. A similar issue of impaired readability exists with articles or products composed substantially of water or liquid.
Experimentation in the industry has shown that such RFID tags are once again readable if there is a substantial air gap interposed between the tag and the article substrate. This required air gap is typically at least one quarter of an inch or greater. FIG. 1 illustrates the relationship between the thickness of the air gap and the measured read range. This figure demonstrates that as the air gap is made larger, the read performance of the RFID tag continues to improve. Because of this well-known relationship between air gap and read performance, various designs have been developed to allow tags to “stand off” from the article substrate in order to create this gap. However, standoff tags are impractical in many commercial applications. The distance created between the tag and the article by the standoff increases the likelihood of the tag being dislodged or damaged in normal use.
Alternately, as noted in the RFID Handbook (Klaus Findenzeller, “RFID Handbook, Second Edition”, John Wiley & sons (2003), p 109.), “by inserting highly permeable ferrite between the [tag antenna] coil and metal surface, it is possible to largely prevent the occurrence of eddy currents. This makes it possible to mount the antenna on [or within] metal surfaces”.
Notwithstanding the reference in the RFID Handbook suggesting the use of highly permeable ferrite, there is little guidance concerning the selection of an appropriate material, nor insight as to a preferred or optimal thickness of the material that should be employed. Therefore, a system for effectively isolating a substrate from an RFID tag, as well as a method for optimizing the performance of RFID tags on the metal substrates would represent a significant advance for the use of 13.56 MHz frequency RFID tags.